The research proposed in this application is focussed on 3 main problems: A. Nuclear transplantation studies of nuclear differentiation. Nuclei of somatic cells show stabilized cell-type specific patterns of gene expression. We propose to determine whether this stabilization is a property of the nucleus, the cytoplasm, or both, by transplanting somatic nuclei into oocytes. Genetic markers now available in the axolotl include genes normally active in oocytes and inactive in most somatic cells (e.g., the gene for melanin synthesis), and genes active in certain somatic cells and not in oocytes (e.g., the gene for ADH in liver). Thus, both positive and negative changes in gene activity can be detected, if they occur, when somatic nuclei are transplanted into oocytes. B. Genetic control of the egg cytoplasm. The egg cytoplasm is known to control early embryonic development of most species. The long range objective of our work on this problem is to identify controlling substances of the cytoplasm, study their synthesis during oogenesis, and their action in controlling development following fertilization. The biological tools required for this work are genes which function during oogenesis to modify or eliminate these substances and thus permit their identification. Five genes of this type are now known in the axolotl and the product of one of them (o ion) has been partially characterized and its role in development analyzed. Further work on presently known genes, and continued search for new genes, are proposed. C. Genetic control of organogenesis: Another long-standing problem concerns the nature of inductive interactions leading to the development of embryonic organs. Genes are known which block the development of the heart (c) and eye (e) in the axolotl. The c gene affects the inducing tissue (endoderm) for heart formation; the e gene effects the competence of the eye-forming ectoderm to respond to induction by chorda-mesoderm. Further analysis of the inductive interactions in normal and mutant embryos is proposed, particularly induction leading to eye formation.